The present invention relates to an electric actuator for transferring a workpiece to a working position by a transfer table driven by an electric motor and more specifically to an electric actuator in which a stepping motor driven by pulses is used as a drive source.
In this type of electric actuator for transferring various workpieces for processing, assembly, and the like, a stepping motor driven by pulses is used as a drive source in some cases. Because a rotation amount of the stepping motor can be controlled according to the number of drive pulses in stages, the stepping motor can perform an excellent function when it is used for accurately setting a transfer stroke of the workpiece or for accurately stopping the workpiece in place.
However, if the stepping motor becomes overloaded, synchronism between the stepping motor and the drive pulses is not maintained and a so-called phenomenon of loss of synchronism is likely to be caused. Therefore, the stepping motor is not suitable for use which requires a constant thrust in a stopped state such as pushing the workpiece against an object with constant pressure and press-fitting the workpiece into a hole with constant force.
By detecting the rotation amount of the stepping motor, a transfer position of the work, and the like and controlling the stepping motor such that the stepping motor stops immediately before it becomes overloaded, it is possible to prevent loss of synchronism and to apply a thrust to the work in the stopped state. However, an extremely complicated and expensive control circuit is required.
It is an object of the present invention to form an electric actuator for transferring a workpiece by using a stepping motor as a drive source such that loss of synchronism of the stepping motor can be prevented and that a thrust can be applied to the workpiece in the stopped state by providing spring means.
To achieve the above object, an electric actuator of the invention comprises a feed screw in a shape of a straight rod rotatably supported by a bearing member on a base, a nut member connected to the feed screw to reciprocate in an axial direction of the feed screw by normal and reverse rotation of the feed screw, a transfer table connected to the nut member with a degree of freedom in the axial direction of the feed screw to transfer a workpiece by moving to follow the nut member, a stepping motor a rotation amount of which is controlled according to the number of drive pulses, a transmission mechanism for transmitting rotating force of the stepping motor to the feed screw, and spring means disposed between the nut member and the transfer table to elastically connect the transfer table and the nut member when the transfer table transfers the workpiece to thereby allow the nut member to overrun, to allow the stepping motor to excessively rotate in a cushioned manner, and to apply a necessary thrust to the transfer table after the transfer table reaches a transfer end of the workpiece.
In the electric actuator having the above structure, after the transfer table reaches the transfer end of the workpiece, the nut member overruns while flexing the spring means, and as a result, the stepping motor also rotates excessively in a cushioned manner. By setting the number of pulses for driving the stepping motor at a value in a range of the excessive rotation, it is possible to prevent poor synchronism due to loss of synchronism between the stepping motor and the drive pulses. By flection of the spring means, it is possible to apply a thrust necessary at a transfer end to the transfer table.
According to a concrete embodiment of the invention, the base has a sectional groove shape with an open upper face, the feed screw extends axially in a central portion of the groove, the nut member has a short pillar shape and is connected to the feed screw to move in the groove in the base, the transfer table has a sectional groove shape and is disposed on an upper face of the base such that the nut member is covered with the transfer table and that the transfer table can move under a guidance of the base, and a connecting member for connecting the transfer table and the nut member is disposed between the transfer table and the nut member such that the connecting member is engage with one of the transfer table and the nut member with a degree of freedom in the axial direction of the feed screw and is engaged with the other with a degree of freedom in a direction orthogonal to an axis of the feed screw.
In the invention, it is preferable that a linear guide mechanism formed of a plurality of balls which can roll is disposed between opposite side walls of the transfer table and opposite groove walls of the base.
According to another concrete embodiment of the invention, the transfer table has a spring receiver through which the feed screw passes on at least axial one end side of the transfer table and the spring means is disposed between the spring receiver and the nut member.
In the invention, it is possible that measuring means for measuring movements of the transfer table and the nut member with respect to each other at the workpiece transfer end is provided to the transfer table. As a result, it is possible to carry out control in which the stepping motor is stopped in response to a signal from a sensor immediately before loss of synchronism when the elastic force of the spring means is set to be weaker than a thrust with which the stepping motor loses synchronism, for example.